[PATCH v2 6/6] libcamera: shared_mem_object: Remove is_standard_layout restriction

[Laurent Pinchart]

On Thu, May 09, 2024 at 12:22:56PM +0100, Kieran Bingham wrote:
> Quoting Laurent Pinchart (2024-05-09 12:16:15)
> > On Thu, May 09, 2024 at 11:49:41AM +0200, Jacopo Mondi wrote:
> > > On Wed, May 08, 2024 at 09:04:01AM GMT, Naushir Patuck wrote:
> > > > The shared_mem_object may be used to construct complex classes, so
> > > > remove the standard layout type restriction.
> > > >
> > > > Signed-off-by: Naushir Patuck <naush at raspberrypi.com>
> > > 
> > > This looks fine to me, but maybe better check on why this was
> > > introduced in first place.
> > 
> > It was added there because sharing objects containing pointers between
> > different processes isn't safe. Naush, what object do you need to store
> > in shared memory that has a non-standard layout, but is still safe to
> > share ?
> 
> Oh, interesting point. I wonder if that's why we've seen people get
> segfaults if the RPi IPA runs in isolation (as then these buffers are
> shared, and could potentially contain invalid pointers?)

On Pi4 the only shared buffer I'm aware of contains the LSC table, so
that should be fine. I haven't checked Pi5.

The std::is_standard_layout() check doesn't catch pointers
unfortunately, but it catches C++ objects that can contain pointers
(such as containers from the C++ standard library). It may also catch
objects that are fine, if that's the case here, we can try to relax the
check.

> > > Reviewed-by: Jacopo Mondi <jacopo.mondi at ideasonboard.com>
> > > 
> > > > ---
> > > >  include/libcamera/internal/shared_mem_object.h | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > >
> > > > diff --git a/include/libcamera/internal/shared_mem_object.h b/include/libcamera/internal/shared_mem_object.h
> > > > index 9b1d939302a8..c9c0482062bd 100644
> > > > --- a/include/libcamera/internal/shared_mem_object.h
> > > > +++ b/include/libcamera/internal/shared_mem_object.h
> > > > @@ -56,7 +56,7 @@ private:
> > > >     Span<uint8_t> mem_;
> > > >  };
> > > >
> > > > -template<class T, typename = std::enable_if_t<std::is_standard_layout<T>::value>>
> > > > +template<class T>
> > > >  class SharedMemObject : public SharedMem
> > > >  {
> > > >  public:

-- 
Regards,

Laurent Pinchart